Sounds like the blow by filter is dirty it should stop if you take that filter off and wash it out good with gasoline, I've had several mowers that would do the same thing wash that filter and I was good to go

I guess the carb I haven't looked at but will next week because he always lets me work on it instead of sending it to the shop.

Start the mower up let it warm up a bit pull the air filter, There is a small rubber hose the size of a dime if that is spraying oil you need to clean or replace the blow by filter I always clean them with gasoline and they work like new hope this help, Jesse